Alzheimer's disease (AD) is characterized by the accumulation of neurofibrillary tangles (NFTs), senile plaques (SPs) and neuropil threads (NTs) in selected telencephalic regions that are especially vulnerable to degeneration. Previous studies have identified a common structural elements in NFTs, NTs and the dystrophic neurites around SPs that are referred to as paired-helical filaments (PHFs). Recently, we showed that a form of SDS-soluble PHFs are composed of A68, a group of putative AD specific proteins. Further, we showed that A68 proteins are abnormally phosphorylated variants of tau, a group of low molecular weight microtubule associated proteins, and that the single KSPV in tau is a phosphate acceptor site in A68 but not in normal human tau. This proposal describes timely experiments to determine how normal tau is transformed into A68, assembled into PHFs and subsequently incorporated into AD NFTs. Specifically, we will determine the extent to which A68 and tau are identical at the amino acid levels in order to exclude the possibility that the primary structure of A68 differs from that of tau. We will elucidate the exact sites of abnormal phosphorylation in A68 that distinguish it from normal tau. We will also define conditions for the in vitro reassembly of highly purified soluble A68 into PHFs. Finally, we will assess whether or not NFT-like structures can be formed by the interaction of A68 PHFs with other suspected protein components of NFTs under defined conditions in vitro. The successful completion of these studies will lead to the complete characterization of A68, the major subunit of PHFs, and they will provide new insight into how PHFs and NFTs are formed from A68 alone or in concert with other neuronal proteins.